The present invention relates to a plant for washing plastic material in scales, in particular a plant of the type destined to be inserted in a line for the treatment of recyclable plastic materials.
The products in plastic material containing one or more recyclable parts are conveyed to the recycling plants and subjected to different treatments to separate the recyclable parts and make them fit for subsequent reuse.
A particular example of recyclable plastic material is the polyethylene terephthalate (PET) of which the bottles containing mineral waters, drinks or similar are generally made.
In order to effectively obtain recyclable PET from these bottles, all those parts joined to the bottle but made with other materials, for instance the labels in paper or plastic, the glue used to stick them to the bottle, as well as the stopper and the base of the bottle, generally made of polyolefin or similar, must be eliminated.
The processes till now proposed in the known technique, comprising different phases of washing and separation of the materials, haven""t given satisfactory results till now because both the purity of the recyclable material so obtained and the costs necessary to get a product of high purity. These drawbacks are mainly due to the need to reconcile two clearly conflicting demands.
On one hand, the washing treatment must be sufficiently energetic and extended in time to allow the separation of the glue fixing the labels. Furthermore, the continuous removal of the glue and the residues of labels from the washing fluid must be guaranteed.
On the other, the costs necessary to satisfy these requirements are particularly high, because of the considerable quantity of washing fluid necessary to remove the glue, and because of the long time, required by the treatment. Furthermore, it must be remembered that it is necessary bring the glue to the melting temperature, to allow the same to melt and to mix with the washing fluid. That requires a high waste of energy, and therefore causes very high costs, if it is desired to obtain recyclable PET of high purity.
The purpose of the present invention is therefore to propose a plant for washing recyclable plastic material in a continuous way that allows the washing of the plastic material, preferably reduced to scales, to be effected in a particular efficient way.
Another purpose of the present invention is to propose a plant of the type specified above, that allows the washing of recyclable plastic material to be effected with particular limited costs.
A further purpose of the present invention is to propose a method for washing the recyclable plastic material in a continuous way that allows recyclable plastic material of high purity to be obtained.
These purposes are achieved by the present invention, which relates to a plant for the continuous washing of plastic material in scales, of the type comprising a washing apparatus equipped with at least one rotating stirrer and containing a washing fluid, at least one filtering unit connected to the apparatus for purifying the washing fluid, a device for feeding the scales to the washing apparatus, a device for withdrawing the scales from the washing apparatus and a plurality of conduits for connecting the washing apparatus and the filtering unit in fluid communication with a circuit in which the washing fluid flows, characterised by comprising means for varying the time in which the scales remain in the washing apparatus as a function of the quantity of scales contained at that same moment in the apparatus.
In this way it is possible to hold the scales in the washing apparatus only for the time necessary to remove all the glue from the scales, thus allowing a considerable limitation of the washing fluid employed during the treatment. The washing fluid is constituted for instance by an aqueous solution maintained at a sufficiently high temperature to melt the glue present on the scales.
The means for varying the time in which the scales remain in the washing apparatus comprise at least a first control device that receives in input a data item representative of the current drawn by is the motor driving the stirrer and controls the operation of the motors connected to the device for feeding the scales and the device for withdrawing the scales respectively. The devices for feeding and withdrawing the scales could be constituted for instance by tilted screw conveyors operated by respective electric motors.
A second control device allows the speed of rotation of the stirrer to be varied as a function of the quantity of scales present at that same moment in the apparatus. That allows advantageously to adapt the frictional action between the scales imparted by the stirrer as a function of their quantity present at a certain instant in the apparatus.
According to a preferential embodiment of the present invention, the washing apparatus is constituted by a closed container that is surrounded by a heat-insulating shell to form a gap between the inside wall of the shell and the external wall of the container.
A heating fluid, for instance high temperature steam, is advantageously introduced into the gap to maintain the inside the apparatus at a constant temperature sufficient to cause the melting of the glue on the scales subjected to treatment and prevent the same glue from solidifying and being deposited again on the scales.
The washing fluid withdrawn from the apparatus is conducted to a filtering unit in which is separated, not only the papery residues still present, but also the glue which is deposited on a fine-pore filtering element. The latter is preferably constituted by a fossil meal filter from which the layer of glue that sticks there is continually removed. The washing fluid as purified is reintroduced by means of a conduit of readmission which carries at least a fraction of the purified washing fluid to the device for withdrawing the scales from the washing apparatus. The purified fluid coming out from the filtering unit, which is undoubtedly cleaner than the washing fluid dragged together with the scales leaving the washing apparatus, is advantageously used to rinse the scales before these go on to a subsequent stage. That makes it possible therefore to bring impurities still present on the scales back into the apparatus again. This not only reduces the quantity of washing fluid employed, but also improves the purity of the material leaving the apparatus.
There is preferably a heat-exchanger provided along the readmission conduit that permits the fluid to be brought up to the desired temperature again before the reintroduction in the washing apparatus, as well as one or more stations to monitor and, if necessary, correct the pH and the composition of the reintroduced fluid.
The invention further concerns a method for washing plastic material in scales, of the type providing the feeding and the withdrawing in a continuous way of scales in a washing apparatus equipped with at least one rotating stirrer and containing a washing fluid, characterised by providing for the regulation of the time the scales remain in the apparatus as a function of the quantity of scales contained at that same moment in the same apparatus.
A particularly advantageous aspect of the method according to the invention consists in maintaining a quantity of washing fluid in the washing apparatus that is proportional to the quantity of scales present at that some moment in the apparatus. This allows that only a quantity of fluid effectively necessary to each instant of the washing phase be used, thus allowing to further limit the consumption of the washing fluid employed.
Furthermore, the method according to the invention, provides the regulation of the speed of rotation of the stirrer as a function of the quantity of scales contained at that same moment in said apparatus, thus allowing the frictional action between the scales to be varied as a function of the actual quantity of scales subjected to washing in any determined instant.